A numerical investigation on thrust and torque production of a batoid fish with asymmetric pectoral fins flapping

Batoid fishes have agile maneuverability by flapping pectoral fins with complex undulatory waves. To reveal the effect of fin motion kinematics on torque production, we numerically study a cownose ray-like swimmer with asymmetrical left and right pectoral fin locomotion in the tethered mode. Our simulation results show that the flow patterns behind the left and right fin differ much due to different motion kinematics. A small flapping amplitude can change the orientation and size of the trailing edge vortices compared to a large flapping amplitude. Flapping frequency is essential to the form of trailing edge vortices which are missing at a low flapping frequency. A too-large wavenumber, thus chordwise deformation of the fin decreases the size and strength of wake vortices. In comparison, the phase difference between the two flapping fins has little effect on the wake structures, and thus, its torque production is limited. Under the parameters studied, a large difference in flapping frequency between the two fins produces the maximum turning torque attributed to the distinctive trailing edge vortices formation. This work may provide insight into the maneuverability control design of fin undulation-based bio-inspired underwater robots/vehicles.